Census Sampling Confusion

Article excerpt

Controversy dogs the use of statistical methods to adjust U.S. population figures

Counting the number of people in the United States is a massive, costly enterprise, done once every 10 years.

For the year 2000 enumeration, the Bureau of the Census plans to mail out more than 90 million questionnaires and deliver millions more by hand to households across the nation before Census Day, April 1. An army of census takers will next fan out across the country aiming to track down the significant number of people who fail to return forms.

Past experience suggests that, despite such a huge effort, the population count will still be incomplete. In 1990, the Census Bureau officially recorded 248,709,873 people. Evidence from other surveys and demographic analyses indicated that the population was closer to 253 million and those not counted were mostly children, people from racial and ethnic minorities, and poor residents of both rural and urban areas.

"Coming out of the 1990 census, we recognized that you can't count everyone by direct enumeration," says Barbara Everitt Bryant, Census Bureau director during that head count.

To obtain the most accurate count possible in the year 2000, the Census Bureau has proposed integrating the results of conventional counting techniques with the results of a large sample survey of the population (SN: 10/11/97, p. 238). Bureau officials call this approach a "one-number census" because the statistically adjusted result will be the one and only official count.

"The plan will help lead to a result that includes more of the overall population, especially for certain subpopulations, and it will help to control costs," Tommy Wright, chief of the Census Bureau's statistical research division, argues in the Jan. 22 SCIENCE.

In January, however, the Supreme Court ruled against the use of statistical sampling methods to obtain population figures for determining how many of the 435 seats in the House of Representatives go to each of the 50 states. At the same time, the court upheld a law that mandates the use of statistically adjusted figures, when feasible, for all other purposes, such as distributing $180 billion in funds for federal programs and determining congressional and state district boundaries.

Last week, the Census Bureau responded to the court's decision and unveiled a revised plan for Census 2000 that would, in effect, provide two population totals. After counting "everyone it possibly can," the bureau would then conduct a quality-check survey, says Kenneth Prewitt, Census Bureau director. The new plan, however, could push the cost of conducting the census from $4 billion to more than $6 billion.

A handful of statisticians has also entered the fray. David A. Freedman of the University of California, Berkeley; Martin T. Wells of Cornell University in Ithaca, N.Y.; and several colleagues contend that current survey techniques don't necessarily provide the accuracy needed to improve census data.

"It's a very difficult problem," Wells says. "There are many, many problems for which statistics is wildly successful, but statistics can't solve everything." Freedman and his colleagues present their critique in a paper posted on the Internet (Technical Report 537 at http:// www.stat.berkeley.edu/tech-reports/).

Many other statisticians disagree with that stand. "You have to look at what else is feasible," says David S. Moore of Purdue University in West Lafayette, Ind. "The real question is, Given the time constraints, the obvious weaknesses in straight enumeration, and the budget Congress is likely to supply, how good a job does the Census Bureau's proposed method do?"

"Generally, the statistical community is very much in favor of trying to do sampling," says Donald B. Rubin of Harvard University.

To illustrate the difficulties of obtaining accurate population figures, Wright uses an analogy. …